Data Sheet

ADP1754/ADP1755

Rev. E | Page 15 of 20

In this example, the worst-case temperature coefficient

(TEMPCO) over −40°C to +85°C is assumed to be 15% for an

X5R dielectric. The tolerance of the capacitor (TOL) is assumed

Substituting these values in Equation 3 yields

Therefore, the capacitor chosen in this example meets the

minimum capacitance requirement of the LDO over temper-

ature and tolerance at the chosen output voltage.

To guarantee the performance of the ADP1754/ADP1755, it is

imperative that the effects of dc bias, temperature, and toler-

ances on the behavior of the capacitors be evaluated for each

application.

UNDERVOLTAGE LOCKOUT

The ADP1754/ADP1755 have an internal undervoltage lockout

circuit that disables all inputs and the output when the input

voltage is less than approximately 1.58 V. This ensures that the

ADP1755/ADP1755 inputs and the output behave in a predicta-

ble manner during power-up.

CURRENT-LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP1754/ADP1755 are protected against damage due to

excessive power dissipation by current-limit and thermal

overload protection circuits. The ADP1754/ADP1755 are

designed to reach current limit when the output load reaches

2 A (typical). When the output load exceeds 2 A, the output

voltage is reduced to maintain a constant current limit.

Thermal overload protection is included, which limits the

junction temperature to a maximum of 150°C (typical). Under

extreme conditions (that is, high ambient temperature and

power dissipation) when the junction temperature begins to

rise above 150°C, the output is turned off, reducing the output

current to zero. When the junction temperature drops below

135°C (typical), the output is turned on again and the output

current is restored to its nominal value.

Consider the case where a hard short from VOUT to ground

occurs. At first, the ADP1754/ADP1755 reach current limit so

that only 2 A is conducted into the short. If self-heating of the

junction becomes great enough to cause its temperature to

rise above 150°C, thermal shutdown activates, turning off the

output and reducing the output current to zero. As the junction

temperature cools and drops below 135°C, the output turns on

and conducts 2 A into the short, again causing the junction

temperature to rise above 150°C. This thermal oscillation between

135°C and 150°C causes a current oscillation between 2A and

0 A that continues as long as the short remains at the output.

Current-limit and thermal overload protections are intended to

protect the device against accidental overload conditions. For

reliable operation, device power dissipation should be externally

limited so that junction temperatures do not exceed 125°C.

THERMAL CONSIDERATIONS

To guarantee reliable operation, the junction temperature of the

ADP1754/ADP1755 must not exceed 125°C. To ensure that the

junction temperature stays below this maximum value, the user

needs to be aware of the parameters that contribute to junction

temperature changes. These parameters include ambient temp-

erature, power dissipation in the power device, and thermal

value is dependent on the package assembly compounds used

and the amount of copper to which the GND pin and the exposed

pad (EPAD) of the package are soldered on the PCB. Table 6 shows

01

130

100

80

500

69

1000

54

6400

42

1

Device soldered to minimum size pin traces.

100

32.7

500

31.5

1000

25.5

The junction temperature of the ADP1754/ADP1755 can be

calculated from the following equation:

(4)

where:

(5)

where:

Power dissipation due to ground current is quite small and can

be ignored. Therefore, the junction temperature equation can

be simplified as follows:

(6)

As shown in Equation 6, for a given ambient temperature, input-

to-output voltage differential, and continuous load current, a

minimum copper size requirement exists for the PCB to ensure

that the junction temperature does not rise above 125°C.

Figure 36 through Figure 41 show junction temperature

calculations for different ambient temperatures, load currents,